Light is difficult to understand because of its dualistic nature and we
need to know just a little bit about light to develop a conceptual
understanding of dyes.

A "photon" is the smallest amount of light. In a way, a photon is to a
light beam what a raindrop is to rain and, sometimes, photons
actually behave just like tiny particles. On the other hand, a photon also
behaves like a "wave" and this behavior is much harder to
explain. Fortunately, all we need to know right now about the wave nature
of light, is that the color of the light is related to the frequency
of the waves.

Let's get some idea about the magnitude of the frequency by looking at the
rainbow spectrum. For example, photons of violet light have a
frequency of 750 TeraHertz. One TeraHertz means 1012 Hertz, that is,
whatever it is that vibrates in the photon, "it" vibrates up and
down on the order of one hundred trillion times per second. At the
low-energy side of the rainbow spectrum, red light has a lower
frequency of 375 THz. The higher the frequency of the light, the higher
its energy.

Violet light is on the high-energy side of the visible spectrum. The
energy of ultraviolet light, just outside the visible range on the
high-energy side, is responsible for sun burns. Infrared "light" cannot be
seen, instead it can be felt as heat and has frequencies below
375 THz. IR photons contain less energy than photons of visible light.

One kind of interaction between light and a molecule consists in
absorption. The photon is absorbed and its energy is used to promote the
molecule into a higher energy state. Such an "excited" molecule can return
to its ground state by the reverse process; the molecule falls
back into its lower energy state while emitting a photon of light
(fluorescence and phosphorescence). Normal dyes are materials that allow
for such absorption-emission sequences with photons of visible light. If a
dye absorbs one color, then it reflects light of all other colors
and is the color complementary to the absorbed light. This is simple to
understand based on the three basic colors: red, green and blue.
The dye of a blue dress absorbs orange (red and green), a red tie absorbs
blue-green, and green lawn absorbs purple light (red and blue).

Many of the big chemical companies started out in the "age of the
dyestuffs," the period between 1865 and 1900, which includes the
discovery of the azo dyes and indigo. Dye discovery, development and
fabrication remain one of the pillars of the modern chemical
industry. The art and science of dye making is very mature. Organic
chemists have been able to design complex organic molecules that
are intensely colored and the dye molecules can be fine-tuned to absorb or
emit light of any desired color of the rainbow spectrum.

In my research group, we are interested in a different and novel kind of
"dye" - we are making "nonlinear dyes." We create materials that
interact with light of one color and emit light of a different color - and
best of all-they emit light of a color that is higher in energy than the
source light. We create materials that emit light with twice the energy of
the source light. Shining infrared laser light (very intense
heatwaves) on such a so-called second-order nonlinear optical material
leads to emission of visible light with twice the frequency of the
infrared light. The design of such "nonlinear dyes" is still in its
pioneering stages and fundamental problems remain. Nonlinear dyes are
at the heart of photonics applications. In photonics, technology light is
used as the primary carrier of information. Photonics is thought to
replace much of today's electronics in communications and computing
applications in the near future.

Normal and nonlinear dye molecules share a few features in that they often
are rod-like and have large electrical-dipole moments along the
long molecular axis. For the novel kind of dye, the relative orientation
of the individual dye molecules becomes a key issue. This is very
different from normal dyes. A blue material will be blue no matter what
the orientations of the neighboring molecules happen to be. The
color of all the dye molecules simply adds up for normal dyes.

For the novel kind of dyes, however, the optical effects add up only if
the nonlinear dye molecules are oriented in the same direction;
otherwise the optical effects cancel each other out. This is where the
problem lies. Most dye molecules have large electrical dipole
moments and nature prefers to arrange polar molecules such that
neighboring molecules are oriented in opposite directions. This
orientation problem is indeed fundamental. It has long been considered
impossible to have polar molecules form crystals in which all the
molecules in the crystal are oriented in the same direction. Trying to
make such highly dipole parallel-aligned organic materials seemed
like a worthwhile goal to pursue in academia.

To tackle this problem, we employ a multi-disciplinary approach that
includes mathematical modeling of crystals of dipolar molecules,
theoretical and computational studies to arrive at and to test rational
design concepts, and, most importantly, the experimental realization
of prototypes.

In the mid-90s, we convinced ourselves it was not impossible to realize
such dipole-parallel aligned molecular crystals. In 1995, we made
the first near-perfect dipole parallel-aligned organic molecular crystal
of a nonlinear dye. The second and third prototypes were realized in
1997 and 2000. An improved design resulted in prototypes four and five in
2000.

Aside from the novel idea, truly innovative academic research takes time,
good faith, patience, a supportive environment, and bright and
talented students. Several graduate students worked with me on this
project. They are Grace Chen (Ph.D. Chemistry 1996, Humboldt
post-doctoral fellow in Zčrich, Switzerland, and Heidelberg, Germany), Don
Steiger (Ph.D. Mathematics 1999, University of Illinois,
Urbana-Champaign, and University of California San Diego post-doctorals),
Michael Lewis (Ph.D. Chemistry 2001, Harvard
University), Zhengyu Wu, and Nathan Knotts. Two undergraduate students
also contributed to this effort: Jason Wilbur (BS, Chemistry
1995) and Mitchell Anthamatten (BS, Chemical Engineering 1996, Ph.D.
Chemistry Engineering 2001). We patented these materials and
the patent is being issued this month. We are only beginning to
understand, and the best is yet to come.